The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell [Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995]. Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250–300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families, see, for example, Hanks et al., FASEB J. 1995, 9, 576–596; Knighton et al., Science 1991, 253, 407–414; Hiles et al., Cell 1992, 70, 419–429; Kunz et al., Cell 1993, 73, 585–596; and Garcia-Bustos et al., EMBO J. 1994, 13, 2352–2361.
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events as described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
A family of type III receptor tyrosine kinases including Flt-3, c-Kit, PDGF-receptor and c-Fms play an important role in the maintenance, growth and development of hematopoietic and non-hematopoietic cells. [Scheijen et al., Oncogene, 2002, 21, 3314–3333 and Reilly, British Journal of Haematology, 2002, 116, 744–757]. Flt-3 and c-Kit regulate maintenance of stem cell/early progenitor pools as well the development of mature lymphoid and myeloid cells [Lyman et al., Blood, 1998, 91, 1101–1134]. Both receptors contain an intrinsic kinase domain that is activated upon ligand-mediated dimerization of the receptors. Upon activation, the kinase domain induces autophosphorylation of the receptor as well as the phosphorylation of various cytoplasmic proteins that help propogate the activation signal leading to growth, differentiation and survival. Some of the downstream regulators of Flt-3 and c-Kit receptor signaling include, PLCγ, PI3-kinase, Grb-2, SHIP and Src related kinases [Scheijen et al., Oncogene, 2002, 21, 3314–3333]. Both receptor tyrosine kinases have been shown to play a role in a variety of hematopoietic and non-hematopoietic malignancies. Mutations that induce ligand independent activation of Flt-3 and c-Kit have been implicated acute-myelogenous leukemia (AML), acute lymphocytic leukemia (ALL), mastocytosis and gastrointestinal stromal tumor (GIST). These mutations include single amino acid changes in the kinase domain or internal tandem duplications, point mutations or in-frame deletions of the juxtamembrane region of the receptors. In addition to activating mutations, ligand dependent (autocrine or paracrine) stimulation of over-expressed wild-type Flt-3 or c-Kit can contribute to the malignant phenotype [Scheijen et al., Oncogene, 2002, 21, 3314–3333].
c-Fms encodes for macrophage colony stimulating factor receptor (M-CSF-1R) which is expressed predominately in the monocytes/macrophage lineage [Dai et al., Blood, 2002, 99, 111–120]. MCSF-1R and its ligand regulate macrophage lineage growth and differentiation. Like the other family members, MCSF-1R contains an intrinsic kinase domain that is activated upon ligand-induced dimerization of the receptor. MCSF-1R is is also expressed in non-hematopoietic cells including mammary gland epithelial cells and neurons. Mutations in this receptor are potentially linked to myeloid leukemias and its expression is correlated with metastatic breast, ovarian and endometrial carcinomas [Reilly, British Journal of Haematology, 2002, 116, 744–757 and Kacinski, Mol. Reprod and Devel., 1997, 46, 71–74]. Another possible indication for antagonists of MCSF-1R is osteoporosis [Teitelbaum, Science 2000, 289, 1504–1508.
PDGF-receptor (PDGFR) has two subunits-PDGFR-α and PDGRR-β, that can form homo or heterodimers upon ligand binding. There are several PDGF ligands: AB, BB, CC and DD. PDGFR is expressed on early stem cells, mast cells, myeloid cells, mesenchymal cells and smooth muscle cells [Scheijen et al., Oncogene, 2002, 21, 3314–3333]. Only PDGFR-β has been implicated in myeloid leukemias—usually as a translocation partner with Tel, Huntingtin interacting protein (HIPI) or Rabaptin5. Recently it was shown that activation mutations in PDGFR-α kinase domain are in gastrointestinal stromal tumors (GIST) [Heinrich et al., Sciencexpress, 2003]
Cyclin-dependent kinases (CDKs) are serine/threonine protein kinases consisting of a β-sheet rich amino-terminal lobe and a larger carboxy-terminal lobe that is largely α-helical. The CDKs display the 11 subdomains shared by all protein kinases and range in molecular mass from 33 to 44 kD. This family of kinases, which includes CDK1, CKD2, CDK4, and CDK6, requires phosphorylation at the residue corresponding to CDK2 Thr160 in order to be fully active [Meijer, Drug Resistance Updates 2000, 3, 83–88].
Each CDK complex is formed from a regulatory cyclin subunit (e.g., cyclin A, B1, B2, D1, D2, D3, and E) and a catalytic kinase subunit (e.g., CDK1, CDK2, CDK4, CDK5, and CDK6). Each different kinase/cyclin pair functions to regulate the different and specific phases of the cell cycle known as the G1, S, G2, and M phases [Nigg, Nature Reviews 2001, 2, 21–32; Flatt et al., Drug Metabolism Reviews 2000, 32, 283–305].
The CDKs have been implicated in cell proliferation disorders, particularly in cancer. Cell proliferation is a result of the direct or indirect deregulation of the cell division cycle and the CDKs play a critical role in the regulation of the various phases of this cycle. For example, the over-expression of cyclin D1 is commonly associated with numerous human cancers including breast, colon, hepatocellular carcinomas and gliomas [Flatt et al., Drug Metabolism Reviews 2000, 32, 283–305]. The CDK2/cyclin E complex plays a key role in the progression from the early G1 to S phases of the cell cycle and the overexpression of cyclin E has been associated with various solid tumors. Therefore, inhibitors of cyclins D1, E, or their associated CDKs are useful targets for cancer therapy [Kaubisch et al., The Cancer Journal 2000, 6, 192–212].
CDKs, especially CDK2, also play a role in apoptosis and T-cell development. CDK2 has been identified as a key regulator of thymocyte apoptosis [Williams et al., European Journal of Immunology 2000, 709–713]. Stimulation of CDK2 kinase activity is associated with the progression of apoptosis in thymocytes, in response to specific stimuli. Inhibition of CDK2 kinase activity blocks this apoptosis resulting in the protection of thymocytes.
In addition to regulating the cell cycle and apoptosis, the CDKs are directly involved in the process of transcription. Numerous viruses require CDKs for their replication process. Examples where CDK inhibitors restrain viral replication include human cytomegakovirus, herpes virus, and varicella-zoster virus [Meijer, Drug Resistance Updates 2000, 3, 83–88].
Inhibition of CDK is also useful for the treatment of neurodegenerative disorders such as Alzheimer's disease. The appearance of Paired Helical Filaments (PHF), associated with Alzheimer's disease, is caused by the hyperphosphorylation of Tau protein by CDK5/p25 [Meijer, Drug Resistance Updates, 2000 3, 83–88].
Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology 2000, 7, 793–803; and Kim and Kimmel, Curr. Opinion Genetics Dev., 2000 10, 508–514]. GSK-3 has been implicated in various diseases including diabetes, Alzheimer's disease, CNS disorders such as manic depressive disorder and neurodegenerative diseases, and cardiomyocyte hypertrophy [PCT Application Nos.: WO 99/65897 and WO 00/38675; and Haq et al., J. Cell Biol. 2000, 151, 117–130]. These diseases are associated with the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role. GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These proteins include glycogen synthase, which is the rate limiting enzyme necessary for glycogen synthesis, the microtubule associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse protein targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation, and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. Along this pathway, GSK-3 is a negative regulator of the insulin-induced signal. Normally, the presence of insulin causes inhibition of GSK-3 mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS 1996, 93, 8455–8459; Cross et al., Biochem. J. 1994, 303, 21–26); Cohen, Biochem. Soc. Trans. 1993, 21, 555–567; and Massillon et al., Biochem J. 1994, 299, 123–128]. However, in a diabetic patient, where the insulin response is impaired, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and long-term effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that in patients with type II diabetes, GSK-3 is overexpressed [see, PCT Application: WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore potentially useful for treating diabetic patients suffering from an impaired response to insulin.
GSK-3 activity is associated with Alzheimer's disease. This disease is characterized by the well-known β-amyloid peptide and the formation of intracellular neurofibrillary tangles. The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 is known to phosphorylate these abnormal sites in cell and animal models. Furthermore, inhibition of GSK-3 has been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Current Biology 1994, 4, 1077–86; and Brownlees et al., Neuroreport 1997, 8, 3251–55]. Therefore, GSK-3 activity promotes generation of the neurofibrillary tangles and the progression of Alzheimer's disease.
Another substrate of GSK-3 is β-catenin, which is degradated after phosphorylation by GSK-3. Reduced levels of β-catenin have been reported in schizophrenic patients and have also been associated with other diseases related to increase in neuronal cell death [Zhong et al., Nature 1998, 395, 698–702; Takashima et al., PNAS 1993, 90, 7789–93; and Pei et al., J. Neuropathol. Exp 1997, 56, 70–78].
GSK-3 activity is associated with stroke [Wang et al., Brain Res 2000, 859, 381–5; Sasaki et al., Neurol Res 2001, 23, 588–92; Hashimoto et al., J. Biol. Chem 2002, 277, 32985–32991].
Another kinase family of particular interest is the Src family of kinases. These kinases are implicated in cancer, immune system dysfunction and bone remodeling diseases. For general reviews, see Thomas and Brugge, Annu. Rev. Cell Dev. Biol. 1997, 13, 513; Lawrence and Niu, Pharmacol. Ther. 1998, 77, 81; Tatosyan and Mizenina, Biochemistry (Moscow) 2000, 65, 49; and Boschelli et al., Drugs of the Future 2000, 25(7), 717.
Members of the Src family include the following eight kinases in mammals: Src, Fyn, Yes, Fgr, Lyn, Hck, Lck, and Blk. These are nonreceptor protein kinases that range in molecular mass from 52 to 62 kD. All are characterized by a common structural organization that is comprised of six distinct functional domains: Src homology domain 4 (SH4), a unique domain, SH3 domain, SH2 domain, a catalytic domain (SH1), and a C-terminal regulatory region [Tatosyan et al., Biochemistry (Moscow) 2000, 65, 49–58].
Based on published studies, Src kinases are considered as potential therapeutic targets for various human diseases. Mice that are deficient in Src develop osteopetrosis, or bone build-up, because of depressed bone resorption by osteoclasts. This suggests that osteoporosis resulting from abnormally high bone resorption can be treated by inhibiting Src [Soriano et al., Cell 1992, 69, 551 and Soriano et al., Cell 1991, 64, 693].
Suppression of arthritic bone destruction has been achieved by the overexpression of CSK in rheumatoid synoviocytes and osteoclasts [Takayanagi et al., J. Clin. Invest. 1999, 104, 137]. CSK, or C-terminal Src kinase, phosphorylates and thereby inhibits Src catalytic activity. This implies that Src inhibition may prevent joint destruction that is characteristic in patients suffering from rheumatoid arthritis [Boschelli et al., Drugs of the Future 2000, 25(7), 717].
Src also plays a role in the replication of hepatitis B virus. The virally encoded transcription factor HBx activates Src in a step required for propagation of the virus [Klein et al., EMBO J. 1999, 18, 5019; and Klein et al., Mol. Cell. Biol. 1997, 17, 6427].
A number of studies have linked Src expression to cancers such as colon, breast, hepatic and pancreatic cancer, certain B-cell leukemias and lymphomas [Talamonti et al., J. Clin. Invest. 1993, 91, 53; Lutz et al., Biochem. Biophys. Res. 1998 243, 503; Rosen et al., J. Biol. Chem. 1986, 261, 13754; Bolen et al., Proc. Natl. Acad. Sci. USA 1987, 84, 2251; Masaki et al., Hepatology 1998, 27, 1257; Biscardi et al., Adv. Cancer Res. 1999, 76, 61; and Lynch et al., Leukemia, 1993, 7, 1416]. Furthermore, antisense Src expressed in ovarian and colon tumor cells has been shown to inhibit tumor growth [Wiener et al., Clin. Cancer Res., 1999, 5, 2164; and Staley et al., Cell Growth Diff, 1997, 8, 269].
Other Src family kinases are also potential therapeutic targets. Lck plays a role in T-cell signaling. Mice that lack the Lck gene have a poor ability to develop thymocytes. The function of Lck as a positive activator of T-cell signaling suggests that Lck inhibitors may be useful for treating autoimmune disease such as rheumatoid arthritis [Molina et al., Nature, 1992, 357, 161]. Hck, Fgr and Lyn have been identified as important mediators of integrin signaling in myeloid leukocytes [Lowell et al., J. Leukoc. Biol., 1999, 65, 313]. Inhibition of these kinase mediators may therefore be useful for treating inflammation [Boschelli et al., Drugs of the Future 2000, 25(7), 717].
Syk is a tyrosine kinase that plays a critical role in Fc□RI mediated mast cell degranulation and eosiniphil activation. Accordingly, Syk kinase is implicated in various allergic disorders, in particular asthma. It has been shown that Syk binds to the phosphorylated gamma chain of the Fc□RI receptor via N-terminal SH2 domains and is essential for downstream signaling [Taylor et al., Mol. Cell. Biol. 1995, 15, 4149].
Inhibition of eosinophil apoptosis has been proposed as key mechanisms for the development of blood and tissue eosinophilia in asthma. IL-5 and GM-CSF are upregulated in asthma and are proposed to cause blood and tissue eosinophilia by inhibition of eosinophil apoptosis. Inhibition of eosinophil apoptosis has been proposed as a key mechanism for the development of blood and tissue eosinophilia in asthma. It has been reported that Syk kinase is required for the prevention of eosinophil apoptosis by cytokines (using antisense) [Yousefi et al., J. Exp. Med. 1996, 183, 1407].
The role of Syk in FcγR dependent and independent response in bone marrow derived macrophages has been determined by using irradiated mouse chimeras reconstituted with fetal liver cells from Syk −/− embryos. Syk deficient macrophages were defective in phagocytosis induced by FcγR but showed normal phagocytosis in response to complement [Kiefer et al., Mol. Cell Biol. 1998, 18, 4209]. It has also been reported that aerosolized Syk antisense suppresses Syk expression and mediator release from macrophages [Stenton et al., J. Immunology 2000, 164, 3790].
The Janus kinases (JAK) are a family of tyrosine kinases consisting of JAK1, JAK2, JAK3 and TYK2. The JAKs play a critical role in cytokine signaling. The down-stream substrates of the JAK family of kinases include the signal transducer and activator of transcription (STAT) proteins. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis as well as in solid and hematologic malignancies such as leukemias and lymphomas. The pharmaceutical intervention in the JAK/STAT pathway has been reviewed [Frank Mol. Med. 5, 432–456 (1999) & Seidel, et al, Oncogene 19, 2645–2656 (2000)].
JAK1, JAK2, and TYK2 are ubiquitously expressed, while JAK3 is predominantly expressed in hematopoietic cells. JAK3 binds exclusively to the common cytokine receptor gamma chain (γc) and is activated by IL-2, IL-4, IL-7, IL-9, and IL-15. The proliferation and survival of murine mast cells induced by IL-4 and IL-9 have, in fact, been shown to be dependent on JAK3- and γc-signaling [Suzuki et al, Blood 96, 2172–2180 (2000)].
Cross-linking of the high-affinity immunoglobulin (Ig) E receptors of sensitized mast cells leads to a release of proinflammatory mediators, including a number of vasoactive cytokines resulting in acute allergic, or immediate (type I) hypersensitivity reactions [Gordon et al, Nature 346, 274–276 (1990) & Galli, N. Engl. J. Med., 328, 257–265 (1993)]. A crucial role for JAK3 in IgE receptor-mediated mast cell responses in vitro and in vivo has been established [Malaviya, et al, Biochem. Biophys. Res. Commun. 257, 807–813 (1999)]. In addition, the prevention of type I hypersensitivity reactions, including anaphylaxis, mediated by mast cell-activation through inhibition of JAK3 has also been reported [Malaviya et al, J. Biol. Chem. 274,27028–27038 (1999)]. Targeting mast cells with JAK3 inhibitors modulated mast cell degranulation in vitro and prevented IgE receptor/antigen-mediated anaphylactic reactions in vivo.
A recent study described the successful targeting of JAK3 for immune suppression and allograft acceptance. The study demonstrated a dose-dependent survival of Buffalo heart allograft in Wistar Furth recipients upon administration of inhibitors of JAK3 indicating the possibility of regulating unwanted immune responses in graft versus host disease [Kirken, Transpl. Proc. 33, 3268–3270 (2001)].
IL-4-mediated STAT-phosphorylation has been implicated as the mechanism involved in early and late stages of rheumatoid arthritis (RA). Up-regulation of proinflammatory cytokines in RA synovium and synovial fluid is a characteristic of the disease. It has been demostrated that IL-4-mediated activation of IL-4/STAT pathway is mediated through the Janus Kinases (JAK 1 & 3) and that IL-4-associated JAK kinases are expressed in the RA synovium [Muller-Ladner, et al, J. Immunol. 164, 3894–3901 (2000)].
Familial amyotrophic lateral sclerosis (FALS) is a fatal neurodegenerative disorder affecting about 10% of ALS patients. The survival rates of FALS mice were increased upon treatment with a JAK3 specific inhibitor. This suggested that JAK3 plays a role in FALS [Trieu, et al, Biochem. Biophys. Res. Commun. 267, 22–25 (2000)].
Signal transducer and activator of transcription (STAT) proteins are activated by, among others, the JAK family kinases. Results form a recent study suggested the possibility of intervention in the JAK/STAT signaling pathway by targeting JAK family kinases with specific inhibitors for the treatment of leukemia [Sudbeck, et al, Clin. Cancer Res. 5, 1569–1582 (1999)]. JAK3 specific compounds were shown to inhibit the clonogenic growth of JAK3-expressing cell lines DAUDI, RAMOS, LC1;19, NALM-6, MOLT-3 and HL-60.
In animal models, TEL/JAK2 fusion proteins have induced myeloproliferative disorders and in hematopoietic cell lines, introduction of TEL/JAK2 resulted in activation of STAT1, STAT3, STAT5, and cytokine-independent growth [Schwaller, et al, EMBO J. 17, 5321–5333 (1998)].
Inhibition of JAK 3 and TYK 2 abrogated tyrosine phosphorylation of STAT3, and inhibited cell growth of mycosis fungoides, a form of cutaneous T cell lymphoma. These results implicated JAK family kinases in the constitutively activated JAK/STAT pathway that is present in mycosis fungoides [Nielsen, et al, Proc. Nat. Acad. Sci. U.S.A. 94, 6764–6769 (1997)]. Similarly, STAT3, STAT5, JAK1 and JAK2 were demonstrated to be constitutively activated in mouse T cell lymphoma characterized initially by LCK over-expression, thus further implicating the JAK/STAT pathway in abnormal cell growth [Yu, et al, J. Immunol. 159, 5206–5210 (1997)]. In addition, IL-6-mediated STAT3 activation was blocked by an inhibitor of JAK, leading to sensitization of myeloma cells to apoptosis [Catlett-Falcone, et al, Immunity 10 ,105–115 (1999)].
The AGC sub-family of kinases phosphorylate their substrates at serine and threonine residues and participate in a variety of well-known signaling processes, including, but not limited to cyclic AMP signaling, the response to insulin, apoptosis protection, diacylglycerol signaling, and control of protein translation (Peterson et al., Curr. Biol. 1999, 9, R521). This sub-family includes PKA, PKB (c-Akt), PKC, PRK1, 2, p70S6K, and PDK.
AKT (also known as PKB or Rac-PK beta), a serine/threonine protein kinase, has been shown to be overexpressed in several types of cancer and is a mediator of normal cell functions [(Khwaja, A., Nature 1999, 401, 33–34); (Yuan, Z. Q., et al., Oncogene 2000, 19, 2324–2330); (Namikawa, K., et al., J. Neurosci. 2000, 20, 2875–2886,)]. AKT comprises an N-terminal pleckstrin homology (PH) domain, a kinase domain and a C-terminal “tail” region. Three isoforms of human AKT kinase (AKT-1, -2 and -3) have been reported so far [(Cheng, J. Q., Proc. Natl. Acad. Sci. USA 1992, 89, 9267–9271); (Brodbeck, D. et al., J. Biol. Chem. 1999, 274, 9133–9136)]. The PH domain binds 3-phosphoinositides, which are synthesized by phosphatidyl inositol 3-kinase (PI3K) upon stimulation by growth factors such as platelet derived growth factor (PDGF), nerve growth factor (NGF) and insulin-like growth factor (IGF-1) [(Kulik et al., Mol. Cell. Biol., 1997, 17, 1595–1606,); (Hemmings, B. A., Science, 1997, 275, 628–630)]. Lipid binding to the PH domain promotes translocation of AKT to the plasma membrane and facilitates phosphorylation by another PH-domain-containing protein kinases, PDK1 at Thr308, Thr309, and Thr305 for the AKT isoforms 1, 2 and 3, respectively. A second, as of yet unknown, kinase is required for the phosphorylation of Ser473, Ser474 or Ser472 in the C-terminal tails of AKT-1, -2 and -3 respectively, in order to yield a fully activated AKT enzyme.
Once localized to the membrane, AKT mediates several functions within the cell including the metabolic effects of insulin (Calera, M. R. et al., J. Biol. Chem. 1998, 273, 7201–7204) induction of differentiation and/or proliferation, protein synthesis and stress responses (Alessi, D. R. et al., Curr. Opin. Genet. Dev. 1998, 8, 55–62,).
Manifestations of altered AKT regulation appear in both injury and disease, the most important role being in cancer. The first account of AKT was in association with human ovarian carcinomas where expression of AKT was found to be amplified in 15% of cases (Cheng, J. Q. et al., Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 9267–9271). It has also been found to be overexpressed in 12% of pancreatic cancers (Cheng, J. Q. et al., Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3636–3641). It was demonstrated that AKT-2 was over-expressed in 12% of ovarian carcinomas and that amplification of AKT was especially frequent in 50% of undifferentiated tumours, suggesting that AKT may also be associated with tumour aggressiveness (Bellacosa, et al., Int. J. Cancer 1995, 64, 280–285).
PKA (also known as cAMP-dependent protein kinase) has been shown to regulate many vital functions including energy metabolism, gene transcription, proliferation, differentiation, reproductive function, secretion, neuronal activity, memory, contractility and motility (Beebe, S. J., Semin. Cancer Biol. 1994, 5, 285–294). PKA is a tetrameric holoenzyme, which contains two catalytic subunits bound to a homo-dimeric regulatory subunit (which acts to inhibit the catalytic sub-units). On binding of cAMP (enzyme activation), the catalytic subunits dissociate from the regulatory subunits to yield the active serine/threonine kinase (McKnight, G. S. et al., Recent Prog. Horm. Res. 1988, 44, pp. 307). Three isoforms of the catalytic subunit (C-α, C-β and C-γ) have been reported to date (Beebe, S. J. et al., J. Biol. Chem. 1992, 267, 25505–25512) with the C-a subunit being the most extensively studied, primarily because of its elevated expression in primary and metastatic melanomas (Becker, D. et al., Oncogene 1990, 5, 1133). To date, strategies to modulate the activity of the C-α subunit involve the use of antibodies, molecules that block PKA activity by targeting regulatory dimers and antisense oligonucleotides expression.
The ribosomal protein kinases p70S6K-1 and -2 are also members of the AGC sub-family of protein kinases and catalyze the phosphorylation and subsequent activation of the ribosomal protein S6, which has been implicated in the translational up-regulation of mRNAs coding for the components of the protein synthetic apparatus. These mRNAs contain an oligopyrimidine tract at their 5′ transcriptional start site, termed a 5′TOP, which has been shown to be essential for their regulation at the translational level (Volarevic, S. et al., Prog. Nucleic Acid Res. Mol. Biol. 2001, 65, 101–186). p70S6K dependent S6 phosphorylation is stimulated in response to a variety of hormones and growth factors primarily via the PI3K pathway (Coffer, P. J. et al., Biochem. Biophys. Res. Commun, 1994 198, 780–786), which may be under the regulation of mTOR, since rapamycin acts to inhibit p70S6K activity and blocks protein synthesis, specifically as a result of a down-regulation of translation of these mRNA's encoding ribosomal proteins (Kuo, C. J. et al., Nature 1992, 358, 70–73).
In vitro PDK1 catalyses the phosphorylation of Thr252 in the activation loop of the p70 catalytic domain, which is indispensable for p70 activity (Alessi, D. R., Curr. Biol., 1998, 8, 69–81). The use of rapamycin and gene deletion studies of dp70S6K from Drosophila and p70S6K1 from mouse have established the central role p70 plays in both cell growth and proliferation signaling.
The 3-phosphoinositide-dependent protein kinase-1 (PDK1) plays a key role in regulating the activity of a number of kinases belonging to the AGC subfamily of protein kinases (Alessi, D. et al., Biochem. Soc. Trans 2001, 29, 1). These include isoforms of protein kinase B (PKB, also known as AKT), p70 ribosomal S6 kinase (S6K) (Avruch, J. et al., Prog. Mol. Subcell. Biol. 2001, 26, 115), and p90 ribosomal S6 kinase (Frodin, M. et al., EMBO J. 2000, 19, 2924–2934). PDK1 mediated signaling is activated in response to insulin and growth factors and as a consequence of attachment of the cell to the extracellular matrix (integrin signaling). Once activated these enzymes mediate many diverse cellular events by phosphorylating key regulatory proteins that play important roles controlling processes such as cell survival, growth, proliferation and glucose regulation [(Lawlor, M. A. et al., J. Cell Sci. 2001, 114, 2903–2910), (Lawlor, M. A. et al., EMBO J. 2002, 21, 3728–3738)]. PDK1 is a 556 amino acid protein, with an N-terminal catalytic domain and a C-terminal pleckstrin homology (PH) domain, which activates its substrates by phosphorylating these kinases at their activation loop (Belham, C. et al., Curr. Biol. 1999, 9, R93–R96). Many human cancers including prostate and NSCL have elevated PDK1 signaling pathway function resulting from a number of distinct genetic events such as PTEN mutations or over-expression of certain key regulatory proteins [(Graff, J. R., Expert Opin. Ther. Targets 2002, 6, 103–113), (Brognard, J., et al., Cancer Res. 2001, 61, 3986–3997)]. Inhibition of PDK1 as a potential mechanism to treat cancer was demonstrated by transfection of a PTEN negative human cancer cell line (U87MG) with antisense oligonucleotides directed against PDK1. The resulting decrease in PDK1 protein levels led to a reduction in cellular proliferation and survival (Flynn, P., et al., Curr. Biol. 2000, 10, 1439–1442). Consequently the design of ATP binding site inhibitors of PDK1 offers, amongst other treatments, an attractive target for cancer chemotherapy.
The diverse range of cancer cell genotypes has been attributed to the manifestation of the following six essential alterations in cell physiology: self-sufficiency in growth signaling, evasion of apoptosis, insensitivity to growth-inhibitory signaling, limitless replicative potential, sustained angiogenesis, and tissue invasion leading to metastasis (Hanahan, D. et al., Cell 2000, 100, 57–70). PDK1 is a critical mediator of the PI3K signalling pathway, which regulates a multitude of cellular function including growth, proliferation and survival. Consequently, inhibition of this pathway could affect four or more of the six defining requirements for cancer progression. As such it is anticipated that a PDK1 inhibitor will have an effect on the growth of a very wide range of human cancers.
Specifically, increased levels of P13K pathway activity has been directly associated with the development of a number of human cancers, progression to an aggressive refractory state (acquired resistance to chemotherapies) and poor prognosis. This increased activity has been attributed to a series of key events including decreased activity of negative pathway regulators such as the phosphatase PTEN, activating mutations of positive pathway regulators such as Ras, and overexpression of components of the pathway itself such as PKB, examples include: brain (gliomas), breast, colon, head and neck, kidney, lung, liver, melanoma, ovarian, pancreatic, prostate, sarcoma, thyroid [(Teng, D. H. et al., Cancer Res., 1997 57, 5221–5225), (Brognard, J. et al., Cancer Res., 2001, 61, 3986–3997), (Cheng, J. Q. et al., Proc. Natl. Acad. Sci. 1996, 93, 3636–3641), (Int. J. Cancer 1995, 64, 280), (Graff, J. R., Expert Opin. Ther. Targets 2002, 6, 103–113), (Am. J. Pathol. 2001, 159, 431)].
Additionally, decreased pathway function through gene knockout, gene knockdown, dominant negative studies, and small molecule inhibitors of the pathway have been demonstrated to reverse many of the cancer phenotypes in vitro (some studies have also demonstrated a similar effect in vivo) such as block proliferation, reduce viability and sensitize cancer cells to known chemotherapies in a series of cell lines, representing the following cancers: pancreatic [(Cheng, J. Q. et al., Proc. Natl. Acad. Sci. 1996, 93, 3636–3641), (Neoplasia 2001, 3, 278)], lung [(Brognard, J. et al., Cancer Res. 2001, 61, 3986–3997), (Neoplasia 2001, 3, 278)], ovarian [(Hayakawa, J. et al., Cancer Res. 2000, 60, 5988–5994), (Neoplasia 2001, 3, 278)], breast (Mol. Cancer Ther. 2002, 1, 707), colon [(Neoplasia 2001, 3, 278), (Arico, S. et al., J. Biol. Chem. 2002, 277, 27613–27621)], cervical (Neoplasia 2001, 3, 278), prostate [(Endocrinology 2001, 142, 4795), (Thakkar, H. et al. J. Biol. Chem. 2001, 276, 38361–38369), (Chen, X. et al., Oncogene 2001, 20, 6073–6083)] and brain (glioblastomas) [(Flynn, P. et al., Curr. Biol. 2000, 10, 1439–1442)].
KDR is a tyrosine kinase receptor that also binds VEGF (vascular endothelial growth factor) Neufeld et al., 1999, FASEB J., 13, 9. The binding of VEGF to the KDR receptor leads to angiogenesis, which is the sprouting of capillaries from preexisting blood vessels. High levels of VEGF are found in various cancers causing tumor angiogenesis and permitting the rapid growth of cancerous cells. Therefore, suppressing VEGF activity is a way to inhibit tumor growth, and it has been shown that this can be achieved by inhibiting KDR receptor tyrosine kinase. For example, SU5416 is a selective inhibitor of the tyrosine kinase and was reported to also suppress tumor vascularization and the growth of multiple tumors. Fong et al., 1999, Cancer Res. 59, 99. Other inhibitors of KDR tyrosine kinase for the treatment of cancer have also been reported (WO 98/54093, WO 99/16755, WO 00/12089).
Examples of cancers that may be treated by such inhibitors include brain cancer, genitourinary tract cancer, lymphatic system cancer, stomach cancer, cancer of the larynx, lung cancer, pancreatic cancer, breast cancer, Kaposi's sarcoma, and leukemia. Other diseases and conditions associated with abnormal tyrosine kinase activity include vascular disease, autoimmune diseases, ocular conditions, and inflammatory diseases.
Aurora-2 is a serine/threonine protein kinase that has been implicated in human cancer, such as colon, breast and other solid tumors. This kinase is involved in protein phosphorylation events that regulate the cell cycle. Specifically, Aurora-2 plays a role in controlling the accurate segregation of chromosomes during mitosis. Misregulation of the cell cycle can lead to cellular proliferation and other abnormalities. In human colon cancer tissue, the aurora-2 protein has been found to be overexpressed [Bischoff et al., EMBO J., 17, 3052–3065 (1998); Schumacher et al., J. Cell Biol., 143, 1635–1646 (1998); Kimura et al., J. Biol. Chem., 272, 13766–13771 (1997)].
Accordingly, there is a great need to develop inhibitors of protein kinases that are useful in treating various diseases or conditions associated with protein kinase activation, particularly given the inadequate treatments currently available for the majority of these disorders.